MSE 3300-Lecture Note 20-Chapter 18 Electrical Properties

8/18/2019 MSE 3300-Lecture Note 20-Chapter 18 Electrical Properties

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MSE 3300 / 5300 UTA Spring 2015 Lecture 20 -

Lecture 20. Electrical Properties

Learning Objectives After this lecture, you should be able to do the following:

1. Describe intrinsic and extrinsic semiconductors (n-type and p-type

semiconductors) with their energy band structures.

2. Understand the conductivity in semiconductors.

Reading

• Chapter 18: Electrical Properties (18.10–15)

Multimedia• Virtual Materials Science & Engineering (VMSE):

http://www.wiley.com/college/callister/CL_EWSTU01031_S/vmse/

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MSE 3300 / 5300 UTA Spr ing 2015 Lecture 20 -

1. Semiconductors

(Intrinsic Semiconductors)

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• Intrinsic semiconductors : the electrical behavior is based on the electronic structure

inherent in the pure material.

• They have a completely filled valence band, separated from an empty conduction band

by a relatively narrow band gap (< 2 eV) at 0 K:• Elemental semiconductors: silicon (Si) and germanium (Ge) have band gap energies of

approximately 1.1 and 0.7 eV.


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MSE 3300 / 5300 UTA Spr ing 2015 Lecture 20 - 3

Charge Carriers in Semiconductors:

Concept of a Hole

Two types of electronic charge carriers:

Free Electron – negative charge (-1.6 X 10-19 C)

– in conduction band

Hole – positive charge (+1.6 X 10-19 C):same magnitude as that for anelectron, but of opposite sign – vacant electron state in

the valence band

Move at different speeds - drift velocities


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Conduction in Semiconductors

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• To become free electrons, electrons must be promoted across the energy band gap and

into empty states at the bottom of the conduction band.

• The excitation energy is from a nonelectrical source such as heat or light.

• Thermal excitation: The number of electrons excited thermally (by heat energy) into the

conduction band depends on the energy band gap width as well as temperature.


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MSE 3300 / 5300 UTA Spring 2015 Lecture 20 - 5

Energy Band Structures:

Insulators & Semiconductors• Insulators:

-- wide band gap (> 2 eV)

-- few electrons excitedacross band gap

Energy

filledband

filledvalence

band

f i l l e d s t a

t e s

GAP

empty

bandconduction

• Semiconductors:-- narrow band gap (< 2 eV)

-- more electrons excitedacross band gap

Energy

filledband

filledvalence

band

f i l l e d s t a

t e s

GAP?

empty

bandconduction


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Conduction in Intrinsic

Semiconductors

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Electron bonding model of

electrical conduction in intrinsic

silicon: (a) before excitation, (b)and (c ) after excitation (the

subsequent free-electron and

hole motions in response to an

external electric field).


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Intrinsic Semiconduction in Terms of

Electron and Hole Migration

electric field electric field electric field

• Electrical Conductivity given by:

# electrons/m

3 electron mobility

# holes/m3

hole mobility

• Concept of electrons and holes:

+-

electron holepair creation

+-

no applied applied

valence

electron Si atom

applied

electron holepair migration


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Number of Charge Carriers

Intrinsic Conductivity

For GaAs ni = 4.8 x 1024 m-3

For Si ni = 1.3 x 1016 m-3

• Ex: GaAs

• for intrinsic semiconductor n = p = ni

σ = ni |e|( μ e + μ h)


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Intrinsic Semiconductors:

Conductivity vs T• Data for Pure Silicon:

-- σ increases with T

-- opposite to metals

Adapted from Fig. 18.16,Callister & Rethwisch 9e.

material

Si

Ge

GaPCdS

band gap (eV)

1.11

0.67

2.252.40

Selected values from Table 18.3,

Callister & Rethwisch 9e.


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Intrinsic Semiconductors

• Pure material semiconductors: silicon (Si) &

germanium (Ge); Band gaps: 1.1 eV and 0.67 eV

– Group IVA materials• Compound semiconductors

– III-V compounds

• Ex: GaAs & InSb; 1.42 eV, 0.17 eV

– II-VI compounds

• Ex: CdS & ZnTe; 2.40 eV, 2.4 eV

– The wider the electronegativity difference between

the elements the wider the energy gap.


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The Periodic Table• Columns: Similar Valence Structure

Adapted fromFig. 2.8,

Callister &

Rethwisch 9e.

Electropositive elements:

Readily give up electrons

to become + ions.

Electronegative elements:

Readily acquire electrons

to become - ions.

g i v e

u p 1 e -

g i v e u

p 2 e -

g

i v e u p 3 e -

i n e r t g a s e s

a c c e p

t 1 e -

a c c e p

t 2 e -

O

Se

Te

Po At

I

Br

He

Ne

Ar

Kr

Xe

Rn

F

ClS

Li Be

H

Na Mg

BaCs

RaFr

CaK Sc

Sr Rb Y


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Table 18.3: Electrical Properties of

Semiconductors

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2. Extrinsic Semiconductors

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• Extrinsic semiconductors : the electrical behavior is determined by impurities.

• An impurity concentration of one atom in 1012 is sufficient to render silicon extrinsic at

room temperature (semiconductor devices: doping with dopants).

n-Type semiconductors p-Type semiconductors

Dopant (donor) : P, As, and Sb Dopant (acceptor) : P, As, and Sb


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n-Type semiconductors: Electron

Energy Band Structure

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n >> p


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p-Type semiconductors: Electron

Energy Band Structure

15

p >> n


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• Intrinsic:-- case for pure Si

-- # electrons = # holes (n = p)

• Extrinsic:-- electrical behavior is determined by presence of impuritiesthat introduce excess electrons or holes

-- n ≠ p

Intrinsic vs Extrinsic Conduction

3+

• p-type Extrinsic: ( p >> n)

no applied

electric field

Boron atom

4+ 4+ 4+ 4+

4+

4+4+4+4+

4+ 4+

hole

• n-type Extrinsic: (n >> p)

no applied

electric field

5+

4+ 4+ 4+ 4+

4+

4+4+4+4+

4+ 4+

Phosphorus atom

valenceelectron

Si atom

conduction

electron

Adapted from Figs. 18.12(a)

& 18.14(a), Callister &Rethwisch 9e.


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Conductivity of Extrinsic

Semiconductors

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n-Type semiconductors p-Type semiconductors


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Conductivity: Temperature

Dependence of Carrier Concentration

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Intrinsic carrier concentration

as a function of temperature

Electron concentration versus

temperature for silicon (n-type)

Solid-state device

operation


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Extrinsic Semiconductors: Conductivity

vs. Temperature• Data for Doped Silicon:-- σ increases doping

-- reason: imperfection sites

lower the activation energy toproduce mobile electrons.

• Comparison: intrinsic vs

extrinsic conduction...-- extrinsic doping level:1021/m3 of a n-type donor

impurity (such as P).

-- for T < 100 K: "freeze-out“,

thermal energy insufficient toexcite electrons.

-- for 150 K < T < 450 K: "extrinsic"

-- for T >> 450 K: "intrinsic"

Adapted from Fig. 18.17, Callister & Rethwisch

9e. (From S. M. Sze, Semiconductor Devices, Physicsand Technology. Copyright © 1985 by Bell Telephone

Laboratories, Inc. Reprinted by permission of John Wiley

& Sons, Inc.)

C o n d u c

t i o n e l e c t r o n

c o n c e n t r a t i o n ( 1 0 2 1 / m 3 )

T (K)6004002000

0

1

2

3

f

r e e z e - o u t

e x t r i n s i c

i n t r i n s i c

doped

undoped


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Conductivity: Factors that Affect

Carrier Mobility

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1. Dopant content

2. Temperature

Dependence of electron and hole mobilities in silicon as a function of the

dopant (both acceptor and donor) content at room temperature


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4. Semiconductor Devices

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• Allows flow of electrons in one direction only (e.g., usefulto convert alternating current to direct current).

• Processing: diffuse P into one side of a B-doped crystal.

-- No applied potential:

no net current flow.

-- Forward bias: carriers

flow through p-type andn-type regions; holes and

electrons recombine at

p-n junction; current flows.

-- Reverse bias: carriers

flow away from p-n junction;

junction region depleted of

carriers; little current flow.

p-n Rectifying Junction

++++

+-

--- -

p-type n-type

+ -

++ +

++

--

--

-

p-type n-type Adapted fromFig. 18.21,

Callister &

Rethwisch

9e.

+++

+

+

---

--

p-type n-type- +


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Properties of Rectifying Junction

Fig. 18.22, Callister & Rethwisch 9e. Fig. 18.23, Callister & Rethwisch 9e.


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MOSFET Transistor

Integrated Circuit Device

• Integrated circuits - state of the art ca. 50 nm line width

– ~ 1,000,000,000 components on chip

– chips formed one layer at a time

• MOSFET (metal oxide semiconductor field effect transistor)


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Summary

1. Semiconductors: intrinsic and extrinsic

semiconductors

2. Band structures of semiconductors3. Conductivity of semiconductors

4. Semiconductor devices

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Homework 10

• 18.4, 18.5, 18.8, 18.11, 18.17

• 18.21, 18.25, 18.29, 18.38

* Problems from Callister, 9th Edition

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MSE 3300-Lecture Note 20-Chapter 18 Electrical Properties